Labeling of a protein, cell, or organism of interest plays a prominent role in many biochemical, molecular biological and medical diagnostic applications. A variety of different labels have been developed and used in the art, including radiolabels, chromolabels, fluorescent labels, chemiluminescent labels, and the like, with varying properties and optimal uses. However, there is continued interest in the development of new labels. Of particular interest is the development of new protein labels, including fluorescent protein labels. Fluorescent proteins or fluoroprotein are proteins that exhibit low, medium or intense fluorescence upon irradiation with light of the appropriate excitation wavelength. The fluorescent characteristic of these proteins is one that arises from the interaction of two or more amino acid residues of the protein, and not from a single amino acid residue. As such, the fluorescent proteins do not include proteins that exhibit fluorescence only from residues that act by themselves as intrinsic fluors, i.e., tryptophan, tyrosine and phenylalanine. As used herein, the term “fluorescent protein” does not include luciferases, such as Renilla luciferase.
Green Fluorescent Protein (GFP), its mutants and homologs are widely known today due to their intensive use as in vivo fluorescent markers in biomedical sciences discussed in detail by Lippincott-Schwartz and Patterson in Science (2003) 300(5616):87-91). The GFP from hydromedusa Aequorea aequorea (synonym A. victoria), discovered by Johnson et al. in J Cell Comp Physiol. (1962), 60:85-104, was found as a part of bioluminescent system of the jellyfish where GFP played role of a secondary emitter transforming blue light from photoprotein aequorin into green light cDNA encoding A. victoria GFP was cloned by Prasher et al. (Gene (1992), 111(2):229-33). It turned out, that this gene can be heterologically expressed in practically any organism due to unique ability of GFP to form fluorophore by itself (Chalfie et al., Science 263 (1994), 802-805). This finding opens broad perspectives for use of GFP in cell biology as a genetically encoded fluorescent label.
The GFP was applied for wide range of applications including the study of gene expression and protein localization (Chalfie et al., Science 263 (1994), 802-805, and Heim et al. in Proc. Nat. Acad. Sci. (1994), 91: 12501-12504), as a tool for visualizing subcellular organelles in cells (Rizzuto et al., Curr. Biology (1995), 5: 635-642), for the visualization of protein transport along the secretory pathway (Kaether and Gerdes, FEBS Letters (1995), 369: 267-271).
A great deal of research is being performed to improve the properties of GFP and to produce GFP reagents useful and optimized for a variety of research purposes. New versions of GFP have been developed, such as a “humanized” GFP DNA, the protein product of which has increased synthesis in mammalian cells (Haas, et al., Current Biology (1996), 6: 315-324; Yang, et al., Nucleic Acids Research (1996), 24: 4592-4593). One such humanized protein is “enhanced green fluorescent protein” (EGFP) mutant variant of GFP having two amino acid substitutions: F64L and S65T (Heim et al., Nature 373 (1995), 663-664). Other mutations to GFP have resulted in blue-, cyan- and yellow-green light emitting versions.
Despite the great utility of GFP, however, other fluorescent proteins with properties similar to or different from GFP would be useful in the art. In particular, benefits of novel fluorescent proteins include fluorescence resonance energy transfer (FRET) possibilities based on new spectra and better suitability for larger excitation. In 1999, GFP homologs were cloned from non-bioluminescent Anthozoa species (Matz et al., Nature Biotechnol. (1999), 17: 969-973). This discovery demonstrated that these proteins are not necessary component of bioluminescence machinery. Anthozoa-derived GFP-like proteins showed great spectral diversity including cyan, green, yellow, red fluorescent proteins and purple-blue non-fluorescent chromoproteins (CPs) (Matz et al., Bioessays (2002), 24(10):953-959). Afterwards, cDNA of GFP homologs were cloned from several Hydroid medusae, including Aequorea macrodactyla (GenBank accession numbers AF435427-AF435433) and Aequorea coerulescens (Gurskaya et al., Biochem J. (2003), 373(Pt 2): 403-408). Thus far, the 40-years history of GFP research revealed GFP-like proteins only within two Cnidaria classes Hydrozoa and Anthozoa.
The utility of fluorescent proteins as a tool in molecular biology has prompted the search for other fluorescent proteins with different and improved properties, as compared to known fluorescent proteins. Thus, it is an object to provide novel fluorescent proteins that exhibit properties not currently available in the limited number of known fluorescent proteins as well as DNAs encoding them that do not suffer from the drawbacks of the known GFP.